Download Geologic time - unconformities

Determining geological ages
◊ Relative age dates – placing rocks and
events in their proper sequence of formation
◊ Numerical dates – specifying the actual
number of years that have passed since an
event occurred (known as absolute age
dating, radiometric dating)
◊ Determining age is difficult
Principles of relative
dating
◊ Law of superposition
◊ Developed by Nicolaus Steno in 1669
◊ In an undeformed or altered sequence
of sedimentary rocks (or layered igneous
rocks), the oldest rocks are on the
bottom
Principles of relative dating - The
law of superposition
Principles of relative
dating
◊ Principle of original horizontality
◊ Layers of sediment are generally deposited in a
horizontal position
◊ Rock layers that are flat have not been
disturbed
Principles of relative
dating
◊ Principle of cross-cutting relationships
• Younger features cut across older feature
◊ Inclusions - a piece of rock that is enclosed within
another rock
• Fault - A break in a rock mass along which
movement has occurred
• Dike - A tubular-shaped intrusive igneous feature
that cuts through the surrounding rock.
UNCONFORMITIES
◊ An unconformity is a break in the rock record produced
by erosion and/or nondeposition of rock units.
These time gaps in the rock layers, are typically on
the order of tens of millions of years or more.
◊ Nondeposition - means that no sediments were
deposited for an interval of time.
◊ uplift and erosion can remove layers deposited at
an earlier time.
◊ Unconformities indicate:
• Major sea level changes
• Major tectonic events have occurred
◊ Types of unconformities
◊ Angular unconformity – tilted rocks are overlain
by flat-lying rocks
◊ Disconformity – strata on either side of the
unconformity are parallel
◊ Nonconformity – metamorphic or igneous rocks
in contact with sedimentary strata
Formation of an
angular unconformity
Nonconformity
◊ A nonconformity is a place where younger sedimentary layers
lie on top of older igneous or metamorphic rocks.
◊ The older igneous or metamorphic rocks indicate active
igneous and/or tectonic period, followed by uplift and erosion
◊ The overlying sedimentary rocks indicate a time of deposition
of sedimentary layers
Nonconformity
DISCONFORMITY
◊ A disconformity is a type of gap in the rock
layers where the above and below an
erosional boundary have the same orientation
(often horizontal)
◊ Disconformities may be difficult to recognize
◊ However, as with all unconformities,
disconformities involve a significant time gap- typically on the order of tens of millions of
years!
Example of missing layers
from a disconformity
DISCONFORMITY
◊ All of the units above the Grand Canyon Series are
horizontal.
◊ Yet, a significant disconformity exists between the
Cambrian Muav Limestone and the Mississippian
Redwall Limestone.